An IO Simulator for Windows Systems

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An I/O Simulator for Windows Systems

• Jalil Boukhobza, Claude Timsit
• jalil.boukhobza_at_prism.uvsq.fr
• 27/10/2004

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Outline

• Introduction
• Overview of the Windows I/O system architecture
• Description of the simulators architecture
  (WinIOSim)
• Whats new in WinIOSim?
• Inputs / Outputs of the simulator
• Validation of WinIOSim
• Summary

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Introduction

• Windows I/O system is poorly studied
• CreateFile() many file access modes
  different caching algorithms big
  performance fluctuations for a given workload.
• Disk subsystems built independently from OS
• Interaction with the OS are not easily
  predictable

What is the performance of a given workload on a
given system architecture for a defined I/O
strategy on Windows systems ?
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Overview of the Windows I/O system architecture

• Different file access modes in the CreateFile()
  function
• Without using the file system cache no buffer
  mode (FILE_FLAG_NO_BUFFERING)
• Using the file system cache sequential, normal,
  write through modes.
• FILE_FLAG_SEQUENTIAL_SCAN, FILE_ATTRIBUTE_NORM
  AL, FILE_FLAG_WRITE_THROUGH

I/O request
FastIO
File System Driver
Cache Manager
Storage Device Driver
Page miss
Storage Device
Virtual Memory Manager
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Description of the WinIOSim architecture (OMNET)
Operating system modules
I/O generator 1
File system cache
I/O generator 2
Process memory
System process
Application process

Disk buffer
I/O scheduler
Disk
I/O generator n
Storage device subsystem modules
Application modules
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The WinIOSim modules

• I/O generators flexible workload generator
• Request types, sizes, numbers, inter arrival
  times
• Access modes, requested addresses, etc.
• Different possible distributions for each
  parameter (poisson, uniform, exponential, etc.)
  thanks to OMNET
• Possible to plug real traces
• Implemented request criticality (synchronous and
  asynchronous requests)
• The process memory and file system cache modules
  simulating the data copy operations, updating
  policies, etc.

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The WinIOSim modules (2)

• The application and system processes
• Request flow control
• Request grouping and splitting
• File system cache prefetching algorithms, lazy
  write and write through algorithms depending on
  access modes
• Both communicate to issue the final request
  sequence (as seen by the disk)
• The different buses simulating bus throughput,
  delays, sharing.

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The WinIOSim modules (3)

• IO scheduler module controls the flow of
  requests to the disk subsystem
• Queuing system FIFO, SCAN, LOOK, SSPF, C-LOOK,
  C-SCAN, etc.
• The disk module very detailed model
• Mapping, zoning, spare area, number of platters,
  seek times, rotational speed, head switching
  times, track and cylinder skew, etc.
• The disk cache module
• Segmentation, read ahead algorithms, lazy write
  and write through algorithms, cache updating
  policies, etc.

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Whats new in WinIOSim?

• Implementation of Windows specific cache
  algorithms depending on access modes identified
  by reverse engineering work
• Specific sequences of data issued by the system
  and application process for each access mode
• Disk subsystem reactions to these algorithms
• Specific reactions for specific sequences
  depending on the disk

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Windows cache / disk cache

• Windows prefetching algorithms No buffer mode
• Read operations
• Sequential mode loading data sequentially Bn,
  Bn1, Bn2, Bn3, etc.
• Normal mode (default) System process

B1
B2
B3
B4
3
3
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What are the disk cache reactions? Will it load
a part of these data ?
The final sequence of request blocks is B1,1,
B1,2, B1,3, B3,1, B2,1, B3,2,B2,2, B3,3, B2,3,
B4,1 , B4,2, ..
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Windows cache / disk cache (2)

• Write operations
• Sequential and normal modes for one request
  some blocks are flushed on the disk and the
  others on the file system cache (later on flushed
  on the disk).
• Write through mode
• Each written block -gt file system cache -gt disk
  cache -gt disk modification of a system file on
  the disk -gt acknowledge.

How will the disk react to those sequences of
blocks to write ?
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Inputs / Outputs of the simulator

• The inputs
• I/O generator configuration
• Modeled by the user
• Real I/O traces
• The simulated architecture definition
• If existing
• Obtained from manufacturers (rarely complete)
• Obtained using the WIOTest parameter extraction
  tool we developed.
• The outputs
• Response times and throughputs (2 main metrics
  for I/Os)
• The different states of the modules at each stage
  of the simulation

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Validation of WinIOSim

• Response time measures / simulations of file
  read/write operation on a sequential file with
  different access modes.
• Less than 4 variation

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Summary

• Efficient tool for Windows I/O system simulation.
• Very accurate and flexible simulations of the
  whole Windows IO system from application to
  disk.
• Simulation of the interactions between the
  modules for example file system cache and disk
  cache.
• Unitary response time validation.

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• Thank you !
• Questions ?
• www.prism.uvsq.fr/users/jboukh
• jalil.boukhobza_at_prism.uvsq.fr